Optimal HR+ Early Breast Cancer Care Must Weigh CDK4/6 Inhibitor Efficacy With Adherence

The rapid evolution of targeted therapies for the management of hormone receptor (HR)–positive, HER2-negative breast cancer has shifted the treatment paradigm, particularly with the introduction of CDK4/6 inhibitors into the adjuvant setting.

During a recent OncLive® Scientific Interchange and Workshop, a panel of experts discussed the nuances of treating patients with high-risk disease, the complexities of selecting between available agents, and the real-world challenges of helping patients maintain treatment adherence.

How does tumor biology drive recurrence risk in early breast cancer?

The workshop began by addressing the risk of recurrence in patients with early-stage disease harboring high-risk features. The experts explained that traditional anatomical staging often fails to capture the full picture of risk, which is why trial enrollment criteria like those used in the phase 3 monarchE study (NCT03155997) focus on biological markers. Patients with 1 to 3 positive nodes (N1) can be categorized as having high-risk disease if they meet additional criteria, such as a tumor size over 5 cm. The panelists reported that data indicate that patients with high-risk N1 disease have a risk of recurrence nearly on par with patients who have N2 or N3 disease, illustrating that biology, rather than just anatomy, is the primary driver of recurrence.

Real-world data involving more than 16,000 patients further demonstrate that patients with node-positive, high-risk features who receive standard-of-care (SOC) endocrine therapy alone still face significant 5-year mortality and recurrence risks.¹ Reshma Mahtani, DO, noted that this understanding has motivated the investigation of adding CDK4/6 inhibitors to adjuvant endocrine therapy to reduce the likelihood of patients developing distant metastatic disease.

Mahtani, who moderated the discussion, is a medical oncologist at Baptist Health Miami Cancer Institute and chief of breast medical oncology at Baptist Health Wellness and Medical Complex in Florida.

What are the key efficacy differences between adjuvant abemaciclib (Verzenio) and ribociclib (Kisqali) in patients with HR-positive, HER2-negative early breast cancer?

Two CDK4/6 inhibitors, abemaciclib and ribociclib, have shown efficacy in the adjuvant setting, although the designs of their respective studies in this setting have notable differences. The monarchE trial established abemaciclib plus endocrine therapy as a SOC for patients with high-risk disease, showing an estimated 7-year invasive disease–free survival (IDFS) rate of 77.4% (n = 2808) vs 70.9% with endocrine therapy alone (n = 2829) at the 7-year follow-up.² Conversely, the phase 3 NATALEE trial (NCT03701334) investigating ribociclib plus endocrine therapy used broader eligibility criteria and included select node-negative patients with high-risk features, showing a 4.5% absolute difference in IDFS rate favoring the abemaciclib arm (n = 2549) over the endocrine therapy alone arm (n = 2552) at 5 years.³

The panelists debated how these data influence their treatment decision-making. One speaker emphasized the weight of the overall survival (OS) data, stating, “the fact that abemaciclib in monarchE has [generated an] OS [benefit] is important, especially when we don’t have any signal of OS coming from NATALEE, and we don’t know if [that is] going to happen because of how broad [that] population is.”

Mahtani added that although the separation of the OS curves in monarchE did not initially show much separation, the benefit with abemaciclib is meaningful for individual patients who are spared from distant recurrence.

How do CDK4/6 inhibitor toxicity profiles and treatment durations influence breast cancer clinical decision-making?

Treatment-emergent adverse effects (TEAEs) and the logistical burden of therapy play a massive role in patient adherence and drug selection, according to the panelists. Abemaciclib is notably associated with gastrointestinal (GI) toxicities, such as diarrhea, whereas ribociclib is associated with risks for neutropenia, elevated liver enzyme levels, and QTc prolongation requiring electrocardiogram (EKG) monitoring. One speaker highlighted the ways that these TEAEs affect the daily lives of many patients, stating, “[abemaciclib is associated with] GI toxicity, including diarrhea, a little bit of grade 3, but quite a bit of grade 2. Grade 2 is [defined as] up to 8 diarrhea events a day. A waitress cannot do this.”

The difference in treatment duration between agents—2 years for abemaciclib vs 3 years for ribociclib^2,3—is also a deciding factor, the experts underscored. They noted that although some oncologists prioritize the shorter duration of abemaciclib to promote greater treatment adherence, others consider ribociclib for patients who do not meet monarchE criteria. The speakers shared their strategies for treating patients with high-risk disease, saying that for CDK4/6 inhibitor–eligible patients who would not have met the monarchE enrollment criteria, ribociclib should be considered.

Beyond clinical factors, the panel also touched on the financial factors that might influence treatment decisions for select patients regarding a 2-year regimen vs a 3-year regimen.

What are the best practices for managing CDK4/6 inhibitor treatment adherence and dose modifications?

The experts explained that ensuring patients continue to receive these therapies requires active management, including dose reductions and supportive care. However, they pointed out that real-world logistics can make this difficult, suggesting that the implementation of a starter-pack dose might be a more practical approach to introducing patients to a consistent and tolerable dose level without needing to adjust dosing based on the time it may take to process a new month’s prescription.

The panelists expressed that they adhere to a low threshold for dose reductions because data from both monarchE and NATALEE indicate that the efficacy of abemaciclib and ribociclib, respectively, is maintained even at lower relative dose intensities. One speaker questioned the high starting doses used in clinical trials, asking, “do we need to go all the way up? If efficacy is similar or same [at lower doses], maybe the companies are dosing it too high and we probably [should start lower]”.

The monitoring requirements for ribociclib also pose challenges. Regarding EKG monitoring, One speaker shared an example experience with a patient where dosing needed to be held for 1 week before EKG results recovered. Despite these hurdles, Mahtani emphasized that providing patients with the full picture of the treatment journey upfront is essential to prevent treatment fatigue.

How is the HR-positive, HER2-negative early breast cancer treatment paradigm evolving in the metastatic setting?

The workshop concluded with a discussion on the metastatic setting, where the use of CDK4/6 inhibitors beyond progression and the emergence of resistance markers like ESR1 mutations are changing clinical practice. Trials such as the phase 2 MAINTAIN (NCT02632045) and phase 3 postMONARCH (NCT05169567) studies have explored switching to a different CDK4/6 inhibitor or continuing treatment beyond initial progression. Furthermore, the introduction of oral selective estrogen receptor degraders like elacestrant (Orserdu) and imlunestrant (Inluriyo) provides new options for patients with ESR1-mutated disease.

A provocative point of discussion was whether to switch therapies based on circulating tumor DNA results before radiographic progression occurs. Mahtani questioned the implications of this shift, saying, “we always talk about waiting for radiographic progression. We don’t change treatment based on tumor markers. We don’t change treatment based on a blood test, but now we have the potential to do that. Are we going to implement it?”

One speaker asked about the practicalities of drug selection based on the backbone therapy, namely tamoxifen or an aromatase inhibitor. Mahtani clarified that although aromatase inhibitors are generally preferred for patients with high-risk disease, the choice of CDK4/6 inhibitor must account for specific risks, such as venous thromboembolism with tamoxifen/abemaciclib or QTc prolongation with tamoxifen/ribociclib.

Ultimately, the panel concluded that although data provide a robust framework of treatment options, individualizing care remains an “art.”

References

1. Reinert T, Pauletto MM, Ferreira FVC, Arenhardt MP. Endocrine therapy in estrogen receptor-low breast cancer: new evidence, persistent questions. Chin Clin Oncol. 2026;15(1):18. doi:10.21037/cco-2025-aw-125
2. Johnston S, Martin M, O'Shaughnessy J, et al. Overall survival with abemaciclib in early breast cancer. Ann Oncol. 2026;37(2):155-165. doi:10.1016/j.annonc.2025.10.005
3. Crown J, Stroyakovskii D, Yardley DA, et al. Adjuvant ribociclib plus nonsteroidal aromatase inhibitor therapy in patients with HR-positive/HER2-negative early breast cancer: 5-year follow-up of NATALEE efficacy outcomes and updated overall survival. ESMO Open. 2025;10(11):105858. doi:10.1016/j.esmoop.2025.105858